Protective coatings for various substrates, including metals, wood, ceramics and paper products are desired to provide resistance to scuffing, abrasion, solvents, foods and soiling. Many coating compositions are solvent-based which may be disadvantageous in use and proper disposal.
Various water-based blends of acrylic polymers and polyurethanes are known for application to various substrates, such as metals, ceramics and wood. For example, water-based coating compositions for base coats or clear coats of metal parts in automobiles are described in U.S. Pat. No. 4,954,559 (Hartog et al).
Such blends have a number of advantages, but also suffer from certain disadvantages, including their thermoplastic nature which limits durability and resistance to common household chemicals (such as detergents, bleach, ammonia), food products and soiling.
Two component coating systems have been produced which provide highly durable coatings which have desired resistance to the household chemicals, food products and soiling. However, these systems may present health hazards, and thus they are not amenable to use for the typical do-it-yourself consumer. Professional application is thus necessary.
One aqueous blend of vinyl monomers and a polyurethane prepolymer is described in U.S. Pat. No. 5,173,526 (Vijayendran et al). The composition includes an oil soluble initiator to cause polymerization of the vinyl monomers during drying. The polyurethane is neutralized with a tertiary amine. There does not appear to be an appreciable amount of self-curing in the vinyl polymer although some crosslinkable monomers are described. The problem with this type of dispersion is that it requires polymerization and chain extension of the polyurethane after mixing, and long curing times unless the curing temperature is raised.
Other compositions having low solvent content are described in the Hartog et al patent noted above. The vinyl polymer is prepared from methylol (meth)acrylamides and other vinyl monomers, including monomers having carboxylic acid groups. There is no suggestion that these dispersions are self-curing, and in preferred embodiments, the polyurethanes can be omitted. These dispersions require some heat for curing and thus are not useful for the typical "do it yourself" consumer.
Still another water-based composition is described in U.S. Pat. No. 4,927,876 (Coogan et al) wherein an aromatic polyurethane is neutralized with a tertiary amine or hydroxide. The vinyl polymer is not self-curing.
U.S. Pat. No. 5,137,961 (Goos et al) relates to a surfactant-free aqueous polymer dispersion containing an anionic vinyl polymer and a polyurethane which is neutralized with triethylamine. Such polyurethanes do not have desired storage stability. Moreover, the composition is not self-curing.
U.S. Pat. No. 5,141,983 (Hasegawa et al) describes the preparation of an aqueous dispersion of a polyurethane and an anionic acrylic copolymer. The polyurethane is neutralized with various materials and the vinyl polymer is prepared by polymerization in the presence of the polyurethane.
In an attempt to provide easy to use coating compositions, considerable development has been carried out to avoid the problems exhibited by known compositions, such as instability upon long term storage, lack of resistance to marring and scuffing and ease of manufacture. Various neutralized polyurethanes have been tried, including those neutralized with triethylamine. Neutralization with these materials has one or more disadvantages, such as poor storage stability of the composition.
Thus, there remains a need for a self-curing, storage stable, water-based coating composition which can be readily used by consumers who like to do home repairs and remodeling and which provides a highly durable and light stable coating on wood or other substrates.